Under federal law, it is a crime for anyone to enter into the US without the approval of an immigration officer -- it's a misdemeanor offense that carries fines and no more than six months in prison.

Many foreign nationals, however, enter the country legally every day on valid work or travel visas, and end up overstaying for a variety of reasons.

But that's not a violation of federal criminal law -- it's a civil violation that gets handled in immigration court proceedings.

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In fact, a 2006 study showed that roughly 45% of undocumented immigrants originally entered the US legally, but then remained in the country without authorization after their visas had expired.

The penalty for this type of violation of immigration law is deportation, and according to the ACLU, "civil removal proceedings far outnumber criminal prosecutions and remain the primary manner in which the federal authorities enforce the immigration laws."

If, however, an undocumented immigrant is deported and then returns to the US without permission, then that "illegal re-entry" constitutes a federal offense with different tiers of accompanying prison time.

While Trump has often focused on immigrants who commit violent felonies while in the US, studies show those are actually the exception -- immigrant crime rates in several instances are actually lower than native US-born populations (with some exceptions).

So although there are more than 11 million unauthorized immigrants living in the US, they haven't all committed a crime just by being in the country.